新疆西昆仑早古生代侵入岩地球化学特征及地质意义
Geochemistry and Geological Significance of Intrusive Rocks of Early Paleozoic in Western Kunlun, Xinjiang
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摘要: 南华纪-早古生代是昆仑山地区洋-陆转换阶段，区域上沿柯岗-库地-其曼于特一带扩张形成古昆仑洋盆。大规模的俯冲消减发生在奥陶纪（481~440 Ma，俯冲型侵入岩发育），志留纪的造山作用结束了洋的演化历程。通过对西昆仑阿喀孜一带侵入岩岩石地球化学特征、LA-ICP-MS锆石U-Pb定年的研究，重塑了本区构造演化过程，将该区早古生代侵入岩细分为晚寒武世壳幔混源序列、晚奥陶世壳幔混源序列和晚志留世壳幔混源序列，从晚寒武世—晚志留世侵入岩显示以下特征：岩石铝饱和指数（A/CNK）值介于（0.86~1.23），具有由偏基性向偏酸性演化趋势；球粒标准化REE配分曲线成近平行曲线簇，稀土总量∑REE（228.96→379.39）显著增高，而δEu（0.79→0.16）降低，表明岩浆分异增大，Eu负异常明显加深，地壳成熟度提高；微量元素显示出富集大离子亲石元素（LILE），亏损高场强元素（HFSE）。结合侵入岩形成的大地构造环境，该期早古生代侵入岩形成于西昆仑地块北缘古昆仑洋盆的俯冲-消减-闭合，塔里木陆块与西昆仑地块陆-陆碰撞造山阶段，为研究该区古昆仑洋消减-消亡构造演化提供了最新的地质资料。Abstract: Nanhua-Early Paleozoic era is the ocean-continent conversion phase in Kunlun Mountains, when the ancient ocean basin was formed by regional expansion along Kegang-Kudi-Qimanyute area. Massive subduction occurred in Ordovician (481-440Ma, subduction-type intrusive rocks developed), and Silurian orogeny ended the oceanic evolution process. Tectonic evolution process of West Kunlun Mountains is reconstructed by studies on the geochemical characteristics of Akazi intrusive rocks as well as its zircon U-Pb dating of LA-ICP-MS research. The Early Paleozoic intrusive rocks can be subdivided into crust-mantle mixed source sequence in late Cambrian, in late Ordovician and in late Silurian. From late Cambrian to late Silurian, intrusive rocks show the following features: firstly, the rock aluminum saturation index(A/CNK) values (0.86→1.23) evolved from more basic to weak acidity; Secondly, chondrite-normalized REE curves show a series of parallel curve clusters, with the amount of REE(228.96→379.39) significantly increasing while δEu (0.79→0.16) decreasing, indicating that the magmatic differentiation grows larger, the negative Eu anomalies increases apparently, and the crustal maturity improve. Besides, trace elements analysis shows enrichment in large ion lithophile elements (LILE), and loss of high field strength elements (HFSE). Combined with geotectonic environment of intrusive rocks formation, the early Paleozoic intrusive rocks were formed in the subduction-extinction-close process of ancient Kunlun Ocean Basin, and in the orogenic stage of collision between Kunlun block and Tarim block. New geological data is thus provided for the study of the subduction-extinction tectonic evolution of ancient Kunlun Ocean in this area.
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